I'm a croatian/slovenian so here goes: Ok so Slovenes, Croatians and Serbs are slavs (more or less) the differences in cultures and languages are pretty minimal. Usually it's nationalistic pride which makes people refuse the idea that we heavily share customs and language. For instance if you spoke Croatian to a Slovenian he would understand 99.9% of the things you said or intertwined Serb to Slovene, Serb to Croat etc.

Anyhow prior to WW2 the 3 peoples have somewhat gotten along pretty well. Than during WW2 everything went to shit mainly between Serbs and Croatians. During WW2 a fascist group officially formed up which was aligned to Axis powers called Ustashe lead by Ante Pavelic who was pretty much on par with Hitler. As you can imagine this did not lead to anything remotely good and it was a pretty big precursor to Yugoslavian wars. Basically while in power Ustashe plain out committed genocide against pretty much everyone that was not a Croat which heavily stuck with Serbs who had their own faction of nutjobs.

It's also proper to note that during WW2 Croatia had a population of about 6-7 million and Ustashe had a very tiny percentage of population supporting them. At the end of the war Croatian state went heavily after Ustashe leadership and core membership but by than some have fled or evaded persecution and by than to much damage has been done to Serb Croatian relationship.

Now put in the mix Muslims which none of the 3 nations really liked and the heavily strained relationship between Serbs and Croatians, Slovenians desire to distance them selves from Balkans and other slav nations and some 40 years of state sponsored repression of ultra nationalism, and heavy financial crisis the state experienced during 80's and you get Yugoslav wars.

The main powerhouse of Yugoslavia was Slovenia (used to be called little Germany) and it was in position it did not really like. Kinda like the position Germany is in right now it pay giving more money than it's getting especially by the end of the Yugoslavia. But Slovenia had relatively less power in the union the federal power almost completely resided in Belgrade. Croatia wanted more power and Slovenia's economy which it for some reason to this envies. Serbia however wanted greater Serbia basically retaining the power it held in Belgrade. And Bosnia was stuck in the middle of the power struggle.

The tensions have been building up more and more and in 90's it all went ape shit insane. Muslims in which were not liked by any of the 3 nations to being with have now been stuck in the middle of a highly ultra nationalistic war. During this war a Serbian criminal formed a militia which consisted mostly of very heavy ultra nationalistic maniacs and criminals who went all genocide on Croatians and Muslims however much less than Muslims, they really went medieval on Muslims. Slovenia mostly is in it's current state because it escaped war thanks to Croatia who would have been in the same state if it did not have to rebuild.

There are many holes left to fill in what I wrote but that would be a rough summary. Anyhow it was all for nothing, all for nothing. I'm not saying we would have been better together but it was all for nothing.

1) Slovenia is not longer the powerhouse it used to be.
2) Croatia is never became what Slovenia is and are still 10 years behind.
3) Serbia eventually lost everything it was fighting for. It lost Bosnia and Kosovo and the federation and set their nation's economy and industry back 30 years.
4) Bosnia which was the most innocent and sort of symbol of brotherhood has been set 30 years behind.

Everything that remained are stupid tensions. Slovenia and Croatia maintained best relations so far but even that is being strained because Croatia is being sort of a cunt and is not willing to give Slovenia a bloody corridor to open international waters and few people easier access to their own country.

Slovenia on the other hand is pining for the few squares of dust land it lost in mutual definition of the border and is sucking up to people who out right dislike us. Slovenia lost 2 massive and I mean massive towns to Austria and Italy we lost huge huge parts of Slovene land to Austria, Italy and Hungary. Slovenes heavily reject the association with it's former sister states stating that they are not a Balkan state which they are, don't have common roots which we do and that difference between Slovenian language and Serbian/Croatian language is same as Chinese and English.

In the end it's a sad story not a single good thing happened after that war. I'll let others fill the blanks but that's about it.